Algimantas Kežionis

The temperature–frequency dependence of dielectric permittivity in Na_0.5Bi_0.5TiO₃ (NBT) -based compositions displays a diffused, frequency-independent maximum along with a frequency-dependent shoulder below this maximum. This behavior deviates from that of both classical ferroelectrics and conventional relaxor ferroelectrics, and its interpretation is further complicated by challenges in linking it to known structural phase transitions. This study proposes a new interpretation of the dielectric behavior of NBT-based materials through a comparative analysis of temperature–frequency permittivity data in both unpoled and poled NBT samples and 0.95Na_0.5Bi_0.5TiO₃–0.05CaTiO₃ solid solution over a broad frequency range (10 Hz–100 MHz). Results reveal that the steep permittivity change between the maximum and shoulder—accompanied by pronounced thermal hysteresis—can be attributed to a phase transition between two non-ferroelectric phases. When this contribution is excluded, the dielectric response aligns with classical relaxor ferroelectric behavior. To reconcile this with other known properties of NBT, the “breathing” model is employed, offering a unified framework for understanding its relaxor-like characteristics.

Comparative studies were performed on variations in the ABO₃ perovskite structure, chemical stability in a CO₂-H₂ gas atmosphere, and electrical conductivity measurements in air, hydrogen, and humidity-involving gas atmospheres of monophase orthorhombic Ba_1−xSr_xCe_0.9Y_0.1O_3−δ samples, where 0 < x < 0.1. The substitution of strontium with barium resulting in Ba_1−xSr_xCe_0.9Y_0.1O_3−δ led to an increase in the specific free volume and global instability index when compared to BaCe_0.9Y_0.1O_3−δ. Reductions in the tolerance factor and cell volume were found with increases in the value of x in Ba_1−xSr_xCe_0.9Y_0.1O_3−δ. Based on the thermogravimetric studies performed for Ba_1−xSr_xCe_0.9Y_0.1O_3−δ, where 0 < x < 0.1, it was found that modified samples of this type exhibited superior chemical resistance in a CO₂ gas atmosphere when compared to BaCe_0.9Y_0.1O_3−δ. The application of broadband impedance spectroscopy enabled the determination of the bulk and grain boundary conductivity of Ba_1−xSr_xCe_0.9Y_0.1O_3−δ samples within the temperature range 25–730 °C. It was found that Ba_0.98Sr_0.02Ce_0.9Y_0.1O_3−δ exhibited a slightly higher grain interior and grain boundary conductivity when compared to BaCe_0.9Y_0.1O_3−δ. The Ba_0.95Sr_0.05Ce_0.9Y_0.1O_3−δ sample also exhibited improved electrical conductivity in hydrogen gas atmospheres or atmospheres involving humidity. The greater chemical resistance of Ba_1−xSr_xCe_0.9Y_0.1O_3−δ, where x = 0.02 or 0.05, in a CO₂ gas atmosphere is desirable for application in proton ceramic fuel cells supplied by rich hydrogen processing gases.